The base64 encoded 128-bit MD5 digest of the message
(without the headers) according to RFC 1864. This header
can be used as a message integrity check to verify that
the data is the same data that was originally sent.

If supplied, after the file has been uploaded to S3,
S3 checks to ensure that the MD5 hash of the uploaded file
matches the hash supplied.

Although it is optional, we recommend using the
Content-MD5 mechanism as an end-to-end integrity check.